The present invention relates to a gelatin containing microgel, oligomers of alpha-gelatin, alpha-gelatin and fractions of alpha-gelatin (peptides), a method for producing such gelatins as well as the use of gelatins according to the invention, particularly as a binder for layers of photographic materials.
Gelatin is known to be a purified protein obtained by partial hydrolysis of the scleroprotein collagen. Due to the different types of raw materials used in the production of gelatin, namely skin material from beef cattle and calves, bacon rind and ossein (demineralized, usually comminuted bone) and due to the differences in the manufacturing processes, the resulting gelatin varies considerably in its chemical and physical properties.
Usually, the raw material is subjected to alkali decomposition, for example by means of milk of lime or soda lye, (the so-called "liming") and then melted out in an essentially neutral solution. Besides the alkali decomposition method, there is also the so-called "acid decomposition method" in which there is no alkali pretreatment and the melting out takes place in an acid medium. The respectively developed gelatin solutions are filtered, concentrated and dried. Gelatin manufacturing methods are discussed in detail in G. Reich, Kollagen, 1966, pages 242 et seq. published by Verlag Theodor Steinkopff, Dresden and in A. G. Ward and A. Courts, Science and Technology of Gelatin, 1977, Academic Press.
The properties of the gelatin thus obtained depend to a great extent on the raw material employed, on the decomposition process selected and especially on the reaction conditions during decomposition, extraction and drying. The methods for producing gelatin and the realization of certain desired properties are based to a large extent on empiric experience. Although it is possible to obtain a considerable degree of reproduceability in this way, it has been found that gelatins intended for the manufacture of photographic products must be tested in practical experiments, in order to enable the manufacture of photographic materials having the required uniformity.
The main component of the raw materials is the so-called tropocollagen, a well defined protein molecule consisting of two identical alpha.sub.1 chains and one alpha.sub.2 chain which is somewhat different. These chains are linked together in the vicinity of their N-terminal amino acid. The amino acid sequence of the alpha.sub.1 chain is well known for the case of calfskin collagen; the polypeptide consists of a linear chain of 1052 amino acids. In this connection, see P. I. Rose & S. Gross, Photographic Gelatin, (published by R. J. Cox), page 89, Academic Press, 1976.
Gelatin consists of a mixture of various fractions of tropocollagen produced during the acid or alkali decomposition. The following four fractions are distinguishable as major components of gelatin and are present in different quantity ratios depending on the origin of the gelatin:
1. alpha-gelatin, consisting of complete alpha polypeptide chains; molecular weight of 9.5.times.10.sup.4 ;
2. oligomers of the alpha chain, consisting of 2 to 15 linked alpha chains; molecular weight of 10.sup.5 -10.sup.6 ;
3. "microgel", consisting of polymers of up to 1000 linked alpha chains; molecular weight 10.sup.7 -10.sup.8 ;
4. "peptides"; different sized cleavage products of the alpha chain; molecular weight 10.sup.4 to 9.times.10.sup.4.
A typical composition of conventional types of gelatin obtained by alkali pretreatment of the raw material and subsequent extraction with water at about 45.degree. to 60.degree. C., is listed in Table 1 below.
TABLE 1 ______________________________________ content Component molecular weight (weight %) ______________________________________ migrogel 10.sup.7 - 10.sup.8 0-15 oligomers of alpha gelatin 10.sup.5 - 10.sup.6 10-30 alpha gelatin 9.5 .times. 10.sup.4 10-40 fragment of alpha gelatin chain (peptides) 10.sup.4 - 9 .times. 10.sup.4 30-80 ______________________________________
In this connection, see A. Veis, The Macromolecular Chemistry of Gelatin, Academic Press 1978, and I. Tomka, Chimia 30, pages 534 et seq. (1976, No. 12). The separation of the gelatin into various fractions has been described in detail by I. Tomka et al in J. Phot. Sci., 23, 97 (1975).